Sediment CH4 Research Articles

Insights into Macroinvertebrate burrowing Activity and Methane Flux in Tidal Flats

Kang, J.; Koo, B.J.; Jeong, K.-S.; Woo, H.J.; Seo, J.; Seo, H.-S.; Kim, M.-S., and Kwon, K., 2018. Insights into macroinvertebrate burrowing activity and methane flux in tidal flats. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 681–685. Coconut Creek (Florida), ISSN 0749-0208.The CH4 flux was examined preliminarily in tidal flats using a circular closed-chamber method to understand the effects of macroinvertebrate burrowing activity. The chamber was deployed over decapods (mud shrimp, Laomedia astacina and crab, Macrophthalmus japonicus) burrows for ~ 2 h, and the CH4 and CO2 concentrations were continuously monitored using a closed, diffuse CH4/CO2 flux meter. It founds that Laomedia astacina burrow (which is relatively long) site afforded higher-level CH4 production, likely due to diffusive emission of CH4 in deep-layer sediments. In addition, the proposed CH4-oxidation process (i.e., reverse methanogenesis) was supported by the decrease in the δ13C of headspace CO2 during the chamber experiment. Therefore, macroinvertebrate burrows appear to be an important ecosystem environment for controlling atmospheric CH4 over tidal flats.

Chromium isotope fractionation during Cr(VI) reduction in a methane-based hollow-fiber membrane biofilm reactor

Chromium (Cr) isotope fractionation analysis is a promising tool for monitoring Cr(VI) reduction in natural aqueous systems. In addition, large amounts of CH4 in natural aqueous sediments are oxidized to CO2 through methanotrophs, thereby mitigating emissions to the atmosphere. However, the investigations on the Cr(VI) reduction process with methanotrophs, and the associated Cr isotope fractionation patterns are scarce. In this study, we have shown that Cr(VI) reduction can occur in the presence of CH4 as the sole electron donor in a hollow-fiber membrane reactor (HfMBR) after direct bacteria enrichment from sediment samples. Products of the methane oxidation by the methanotrophs are used by microbes to reduce Cr(VI) as shown by the progressive increase in δ53Cr with time in the CH4 feed reactor. The isotope fractionation factor (ε) of −2.62 ± 0.20‰ was obtained from the application of the Rayleigh distillation model. The results of Cr isotope fractionation analysis also explained the decrease of Cr(VI) concentration in the N2 feed reactor, where the δ53Cr values remained steady in the first two weeks but significantly increased in the last two weeks, indicating that physical adsorption and subsequent Cr(VI) reduction occurred. This study extended the application of Cr isotope fractionation, showing the suitability of this method for clarifying different Cr(VI) removal processes.

Effects of alternative electron acceptors on the activity and community structure of methane-producing and consuming microbes in the sediments of two shallow boreal lakes

The role of anaerobic CH4 oxidation in controlling lake sediment CH4 emissions remains unclear. Therefore, we tested how relevant EAs (SO42-, NO3-, Fe3+, Mn4+, O2) affect CH4 production and oxidation in the sediments of two shallow boreal lakes. The changes induced to microbial communities by the addition of Fe3+ and Mn4+ were studied using next-generation sequencing targeting the 16S rRNA and methyl-coenzyme M reductase (mcrA) genes and mcrA transcripts. Putative anaerobic CH4-oxidizing archaea (ANME-2D) and bacteria (NC 10) were scarce (up to 3.4% and 0.5% of archaeal and bacterial 16S rRNA genes, respectively), likely due to the low environmental stability associated with shallow depths. Consequently, the potential anaerobic CH4 oxidation (0-2.1 nmol g-1dry weight (DW)d-1) was not enhanced by the addition of EAs, nor important in consuming the produced CH4 (0.6-82.5 nmol g-1DWd-1). Instead, the increased EA availability suppressed CH4 production via the outcompetition of methanogens by anaerobically respiring bacteria and via the increased protection of organic matter from microbial degradation induced by Fe3+ and Mn4+. Future studies could particularly assess whether anaerobic CH4 oxidation has any ecological relevance in reducing CH4 emissions from the numerous CH4-emitting shallow lakes in boreal and tundra landscapes.

Spatial variation in flux, δ13C and δ2H of methane in a small Arctic lake with fringing wetland in western Greenland

Small lakes in northern latitudes represent a significant source of CH4 to the atmosphere that is predicted to increase with warming in the Arctic. Yet, whole-lake CH4 budgets are lacking as are measurements of δ13C-CH4 and δ2H-CH4. In this study, we quantify spatial variability of diffusive and ebullitive fluxes of CH4 and corresponding δ13C-CH4 and δ2H-CH4 in a small, Arctic lake system with fringing wetland in southwestern Greenland during summer. Net CH4 flux was highly variable, ranging from an average flux of 7 mg CH4 m−2 d−1 in the deep-water zone to 154 mg CH4 m−2 d−1 along the lake margin. Diffusive flux accounted for ~8.5 % of mean net CH4 flux, with plant-mediated and ebullitive flux accounting for the balance of the total net flux. Methane content of emitted ebullition was low (mean ± SD 10 ± 17 %) compared to previous studies from boreal lakes and wetlands. Isotopic composition of net CH4 emissions varied widely throughout the system, with δ13C-CH4 ranging from −66.2 to −55.5 ‰, and δ2H-CH4 ranging from −345 to −258 ‰. Carbon isotope composition of CH4 in ebullitive flux showed wider variation compared to net flux, ranging from −69.2 to −49.2 ‰. Dissolved CH4 concentrations were highest in the sediment and decreased up the water column. Higher concentrations of CH4 in the hypoxic deep water coincided with decreasing dissolved O2 concentrations, while methanotrophic oxidation dominated in the epilimnion based upon decreasing concentrations and increasing values of δ13C-CH4 and δ2H-CH4. The most depleted 13C- and 2H-isotopic values were observed in profundal bottom waters and in subsurface profundal sediments. Based upon paired δ13C and δ2H observations of CH4, acetate fermentation was likely the dominant production pathway throughout the system. However, isotopic ratios of CH4 in deeper sediments were consistent with mixing/transition between CH4 production pathways, indicating a higher contribution of the CO2 reduction pathway. The large spatial variability in fluxes of CH4 and in isotopic composition of CH4 throughout a single lake system indicates that the underlying mechanisms controlling CH4 cycling (production, consumption and transport) are spatially heterogeneous. Net flux along the lake margin dominated whole-lake flux, suggesting the nearshore littoral area dominates CH4 emissions in these systems. Future studies of whole-lake CH4 budgets should consider this significant spatial heterogeneity.

Rates and pathways of sedimentary organic matter mineralization in two basins of a boreal lake: Emphasis on methanogenesis and methanotrophy

AbstractSediment porewater was analyzed at several sampling dates in two adjacent basins of an oligotrophic boreal lake, one basin perennially oxygenated (Basin A) and the other occasionally anoxic (Basin B). Depth concentration profiles of methane (CH4), dissolved inorganic carbon (DIC), and electron acceptors were modeled with a one‐dimensional transport‐reaction equation to constrain the depth intervals (zones) where solutes are produced/consumed in the top 10 cm of the sediment column, and to obtain the net reaction rates in each zone. This multicomponent geochemical modeling reveals that CH4 was produced below 4–7 cm depth at lower rates in Basin A (250–800 fmol cm−2 s−1) than in Basin B (1900–6500 fmol cm−2 s−1) and that methanogenesis accounted for 30–64% and 84–100% of the sediment organic matter (OM) mineralization in Basins A and B, respectively. We show that methanogenesis did not always yield equimolar amount of CH4 and DIC, as would be expected from the fermentation of the model molecule CH2O. While ∼50% of the CH4 produced in Basin A is oxidized in the sediment column, this proportion decreases to ∼20% in Basin B. Dioxygen is by far the main electron acceptor for CH4 and OM oxidations in both basins. Methanotrophy in the sediment, however, is not limited to the ∼4‐mm thick surface layer in which O2 diffuses from bottom water but occurs down to 4–7 cm depth where O2 is transported through bioirrigation. Thermodynamic calculations suggest that, in addition to O2, Fe oxyhydroxides, and sulfate may serve as oxidants for methanotrophy in that zone. We predict that Basin B sediments release more CH4 than DIC whereas Basin A sediments mainly export DIC. This study highlights that small changes in hypolimnetic O2 levels may significantly alter the magnitude of OM mineralization pathways and the fate of CH4 in boreal lake sediments.

Influence of run of river dams on floodplain sediments and carbon dynamics

Quantifying the biophysical impacts of management on river and stream ecosystems is an important issue that requires understanding of ecological, hydrological and geomorphological processes. We conducted a year-long ecogeomorphological experiment to determine sedimentation and carbon cycling differences between run-of-river (ROR) dams in a 200year old impounded floodplain and a floodplain that was formerly impounded >65years ago. Our study shows that ROR dams do not necessarily enhance floodplain sedimentation or carbon storage, but promote brief periods of sediment CH4 flux (up to 2.91nmolCH4m−2s−1) to the atmosphere. Removal of a ROR dam may result in channel widening, and removal by lateral transport (i.e., erosion) of nearly 14 MgC per floodplain. We did not find significant differences in mean sediment CO2 fluxes or temperature sensitivity (Q10=2.1±0.4) of CO2 efflux among floodplains. All floodplains were likely an annual net source of sediment CO2 flux (annual mean of 2.12±0.974μmolCO2m−2s−1) to the atmosphere, and a sink for atmospheric CH4 (annual mean of −0.221±0.163nmolCH4m−2s−1). We provide a conceptual model on the management consequences on ROR dam structures for floodplain sedimentation/erosion, and sediment carbon cycling.

Sediment iron content does not play a significant suppressive role on methane emissions from boreal littoral sedge (Carex) vegetation

We studied the effects of Fe(III) additions, sediment iron content and other environmental variables on methane (CH4) emissions from stands of two sedges, Carex acuta and Carex rostrata, originating from the littoral area of two boreal lakes having different total iron 33.1(±1.7) and 8.6(±1.2)gkg−1 (DW) and organic matter content 16.8(±3.1) and 28.2(±2.5)% (of DW) in the sediment, respectively. An outdoor mesocosm experiment was carried out during two ice-free periods. The sediment of mesocosms was treated with amorphous Fe(III) oxyhydroxide (1–2% of the original total iron content). The CH4 emission was measured with the closed-chamber method. The Fe(III) additions did not significantly affect CH4 emissions from either of the Carex stands, although a minor effect on sediment CH4 concentrations could be seen. In both Carex stands the temporal variation of CH4 emission rates was primarily related to sediment CH4 concentration, which was negatively influenced by air pressure and positively by temperature of the preceding week. Despite the greater rhizome iron content, the CH4 emissions from C. acuta stands were greater (1.02–102mg CH4m−2d−1 and 6.68–37.1 CH4m−2d−1 during the first and second growing season, respectively) than that from C. rostrata stands (0.13–16.9 CH4m−2d−1 and 1.30–22.8) CH4m−2d−1, respectively. The iron content of the sediments and rhizomes played a minor role on CH4 emissions from littoral Carex stands. Annual and between-vegetation stand variation in CH4 emissions seemed to be more related to site and taxon-specific differences in plant biomass growth affecting both CH4 production and oxidation in the sediment.

Real-time monitoring of subsurface microbial metabolism with graphite electrodes.

Monitoring in situ microbial activity in anoxic submerged soils and aquatic sediments can be labor intensive and technically difficult, especially in dynamic environments in which a record of changes in microbial activity over time is desired. Microbial fuel cell concepts have previously been adapted to detect changes in the availability of relatively high concentrations of organic compounds in waste water but, in most soils and sediments, rates of microbial activity are not linked to the concentrations of labile substrates, but rather to the turnover rates of the substrate pools with steady state concentrations in the nM–μM range. In order to determine whether levels of current produced at a graphite anode would correspond to the rates of microbial metabolism in anoxic sediments, small graphite anodes were inserted in sediment cores and connected to graphite brush cathodes in the overlying water. Currents produced were compared with the rates of [2-14C]-acetate metabolism. There was a direct correlation between current production and the rate that [2-14C]-acetate was metabolized to 14CO2 and 14CH4 in sediments in which Fe(III) reduction, sulfate reduction, or methane production was the predominant terminal electron-accepting process. At comparable acetate turnover rates, currents were higher in the sediments in which sulfate-reduction or Fe(III) reduction predominated than in methanogenic sediments. This was attributed to reduced products (Fe(II), sulfide) produced at distance from the anode contributing to current production in addition to the current that was produced from microbial oxidation of organic substrates with electron transfer to the anode surface in all three sediment types. The results demonstrate that inexpensive graphite electrodes may provide a simple strategy for real-time monitoring of microbial activity in a diversity of anoxic soils and sediments.

Impact of Hurricane Sandy on CH4 Released from Vegetated and Unvegetated Wetland Microsites

Hurricane Sandy was one of the largest tropical storms to pass over the Atlantic basin, causing destruction along its path as it made landfall in Jamaica, Cuba, the Bahamas, and the United States [Tropical Cyclone Report: Hurricane Sandy; National Weather Service, National Oceanic and Atmospheric Administration, 2012 (http://www.nhc.noaa.gov/data/tcr/AL182012_Sandy)]. Hurricane Sandy passed over the Meadowlands in the midst of our multiyear study on marsh CH4 dynamics, providing a unique opportunity to evaluate the effects of hurricanes on CH4 cycling within an estuarine marsh. We modeled dissolved CH4 distributions in wetland sediments from 2011 (Reid, M. C.; Tripathee, R.; Shäfer, K. V. R.; Jaffé, P. R. Tidal Marsh Methane dynamics: Difference in seasonal lags in emissions driven by storage in vegetated versus unvegetated sediments. J. Geophys. Res.: Biogeosci. 2013, 118, 1802–1813) to 2013 and estimated that Hurricane Sandy did not degas vegetated soils but degassed between 45 and 75% of the dissolved CH4 in unvegetated sediments. Hurricanes may regularly affect coastal wetland CH4 emissions globally, but because these wetland sediments do not store substantial dissolved CH4 late in the year, the degassing of these sediments by Hurricane Sandy did not play an important role in the annual carbon emissions from this marsh.

Magnitude and variability of methane production and concentration in tropical coastal lagoons sediments

AIM: Coastal wetlands are potential zones for methane (CH4) production. The present study aims to evaluate the spatial variation of CH4 production and concentration in ten tropical coastal lagoons, the influence of aquatic macrophytes on the sediment CH4 concentration and how the magnitude of these potential CH4 production rates compare to those in other ecosystems. METHODS: Sediments were sampled in ten coastal lagoons, with one site in the limnetic region and another site in aquatic macrophyte stands when they were present in the littoral region. We measured the CH4 production as a potential rate, and CH4 concentration was directly measured from sediment samples. RESULTS: The highest potential CH4 production (PMP) rates were found in alkaline and hypersaline lagoons. However, Cabiúnas, which is a freshwater lagoon densely colonized by aquatic macrophytes, also exhibited a high PMP rate. We also observed that the sediment CH4 concentration in the littoral region was higher than in the limnetic region in all of the investigated lagoons except Paulista, which presented the opposite pattern. The PMP rates observed in the studied lagoons were low compared to other aquatic ecosystems. CONCLUSION: Our results suggest that the sediments of saline lagoons are important CH4 production sites, and in general, aquatic macrophytes have a positive influence on methanogenesis, which was evident based on the CH4 concentrations present in the sediments of these lagoons. Further studies should focus on the processes underlying the CH4 patterns observed in the tropical coastal lagoons, especially concerning the coupling between CH4 production and concentration.

Biodiversity effects of benthic ecosystem engineers on the spatial patterns of sediment CH4 concentration in an urban Neotropical coastal lagoon

AIM: Biodiversity of sediment bioturbators has been shown to be important for to the magnitude and stability of benthic-pelagic processes. However, no study to date has evaluated the importance of the biodiversity of benthic invertebrate bioturbators to the spatial patterns of sediment CH4 concentration ([CH4]). Here we conducted a laboratorial experiment to test the following predictions: (1) Bioturbator species richness will reduce the sediment [CH4]; (2) individual bioturbator species (i.e. species composition) will have different effects on sediment [CH4]; (3) and both the effects of bioturbator species richness and composition on sediment [CH4] will be dependent on sediment depth. METHODS: We manipulated the number and composition of three functional divergent benthic invertebrate bioturbators species that are widespread in South Atlantic coastal lagoons, in laboratorial sediment chambers containing the sediment and water of an urban impacted coastal lagoon RESULTS: Bioturbator species richness had no overall significant effect on sediment [CH4] when comparisons of sediment [CH4] were made among species richness levels. However, bioturbator species richness significantly reduced sediment [CH4] when species richness levels were compared to the control (defaunated treatments), but this effect was significant only at the deepest sediment layer. Furthermore, bioturbator species composition had significant, but distinct effects on the patterns of reduction in sediment [CH4], depending on the sediment depth and the bioturbator species. CONCLUSIONS: We conclude that both the number and composition of bioturbator species are important to determine the effects of benthic bioturbators on spatial patterns of sediment [CH4], but the strength of these effects depend on species traits that determine interspecific interactions strength across the sediment vertical niche space.

Substrate limitation of sediment methane flux, methane oxidation and use of stable isotopes for assessing methanogenesis pathways in a small arctic lake

Among predicted impacts of climate change in the Arctic are greater thaw depth and shifts in vegetation patterns and hydrology that are likely to increase organic carbon and nutrient loading to lakes. We measured substrate limitation of sediment methane (CH4) flux, examined pathways of methanogenesis, and potential CH4 oxidation using stable isotope labeled acetate in intact sediment cores from arctic lake GTH 112 (68°40′20″N, 149°14′57″W). We hypothesized that the acetoclastic pathway would dominate methanogenesis, reflecting dissolved organic carbon supply from the surrounding landscape, and that sediment CH4 flux would be stimulated by addition of acetate. Experiments demonstrated acetate limitation of sediment CH4 flux with short-term CH4 flux response to availability of acetate, high rates of CH4 oxidation, and strong dominance of the acetoclastic over the hydrogenotrophic methanogenic pathway. The experiments also indicated that isotopic fractionation effects during isotope enrichment experiments are large during methanogenesis and can alter the methanogenic pathways being investigated. Under oxic conditions, CH4 oxidation at the sediment–water interface or in the water column is likely to account for much of diffusive CH4 flux, but under anoxic hypolimnetic conditions and increased substrate availability, conditions that are likely to occur with climate change, sediment CH4 flux will likely increase, with oxidation utilizing a smaller portion of sediment CH4 production.

Spatial variation in shallow sediment methane sources and cycling on the Alaskan Beaufort Sea Shelf/Slope

The MITAS (Methane in the Arctic Shelf/Slope) expedition was conducted during September, 2009 onboard the U.S. Coast Guard Cutter (USCGC) Polar Sea (WAGB-11), on the Alaskan Shelf/Slope of the Beaufort Sea. Expedition goals were to investigate spatial variations in methane source(s), vertical methane flux in shallow sediments (<10 mbsf), and methane contributions to shallow sediment carbon cycling. Three nearshore to offshore transects were conducted across the slope at locations approximately 200 km apart in water column depths from 20 to 2100 m. Shallow sediments were collected by piston cores and vibracores and samples were analyzed for sediment headspace methane (CH4), porewater sulfate (SO42−), chloride (Cl−), and dissolved inorganic carbon (DIC) concentrations, and CH4 and DIC stable carbon isotope ratios (δ13C). Downward SO42− diffusion rates estimated from sediment porewater SO42− profiles were between −15.4 and −154.8 mmol m−2 a−1 and imply a large spatial variation in vertical CH4 flux between transects in the study region. Lowest inferred CH4 fluxes were estimated along the easternmost transect. Higher inferred CH4 flux rates were observed in the western transects. Sediment headspace δ13CCH4 values ranged from −138 to −48‰, suggesting strong differences in shallow sediment CH4 cycling within and among sample locations. Measured porewater DIC concentrations ranged from 2.53 mM to 79.39 mM with δ13CDIC values ranging from −36.4‰ to 5.1‰. Higher down-core DIC concentrations were observed to occur with lower δ13C where an increase in δ13CCH4 was measured, indicating locations with active anaerobic oxidation of methane. Shallow core CH4 production was inferred at the two western most transects (i.e. Thetis Island and Halkett) through observations of low δ13CCH4 coupled with elevated DIC concentrations. At the easternmost Hammerhead transect and offshore locations, δ13CCH4 and DIC concentrations were not coupled suggesting less rapid methane cycling. Results from the MITAS expedition represent one of the most comprehensive studies of methane source(s) and vertical methane flux in shallow sediments of the U.S. Alaskan Beaufort Shelf to date and show geospatially variable sediment methane flux that is highly influenced by the local geophysical environment.

Quantifying rates of methanogenesis and methanotrophy in Lake Kinneret sediments (Israel) using pore‐water profiles

Full seasonal sets of chemical and isotope profiles from the pore water of Lake Kinneret (Sea of Galilee, Israel) were produced to study methanogenesis and methanotrophy processes and the couplings between methane (CH4), sulfur, and iron. Sulfate is depleted within the upper 10 cm of the sediment mainly by traditional bacterial sulfate reduction by organic matter. Maximum sulfate reduction rates calculated from sulfate concentration profiles are found at the water–sediment interface (0–1 cm − 1.4 × 10−12 ± 0.2 × 10−12 mol cm−3 s−1). CH4 concentrations and modeling of dissolved inorganic carbon (DIC) and its stable carbon isotope (δ13CDIC) suggest that maximum methanogenesis rates of 2.5 × 10−13 ± 1.5 × 10−13 mol cm−3 s−1 occur at 5–12‐cm depth in the sediments, and that it ends at 20 cm. Of the produced CH4, 50–75% is converted to gas bubbles of CH4 before it reaches the bottom water. Model results suggest the occurrence of anaerobic oxidation of CH4 (AOM) in the deep sediments of the lake below the zone of methanogenesis.

Benthic bioturbator enhances CH4 fluxes among aquatic compartments and atmosphere in experimental microcosms

We utilized laboratory microcosms to evaluate the effects of a benthic sediment bioturbator ( Heteromastus similis ; Polychaeta; conveyor-belt deposit feeder) on vertical distributions of CH4 in sediment and net CH4 fluxes across sediment–water–air interfaces. The effect of H. similis on sediment CH4 concentration ([CH4]) varied depending on sediment depth and was strongest at higher animal densities. In comparison with defaunated controls, microcosms with the highest density of H. similis exhibited an increase in [CH4] of 3.7-fold, on average, at the sediment surface (0–2 cm), but these concentrations decreased by ~2-fold in deeper sediment layers (2–8 cm). However, irrespective of sediment depth, the density of H. similis resulted in an overall nonlinear reduction of bulk sediment [CH4]. Most of the observed CH4 losses from the sediment were due to CH4 oxidation, but the bioturbatory activities of H. similis also promoted significant increases in [CH4] in both the water column and the microcosm headspace. These results suggest that benthic invertebrates can mediate CH4 turnover between compartments in aquatic ecosystems, with further consequences for the coupling between benthic–pelagic food chains via the methanotrophic-mediated microbial loop, as well as increase CH4 emissions to the atmosphere.

A survey of methane isotope abundance (14C, 13C, 2H) from five nearshore marine basins that reveals unusual radiocarbon levels in subsurface waters

Methane (CH4) in the subsurface ocean is often supersaturated compared to equilibrium with the modern atmosphere. In order to investigate sources of CH4 to the subsurface ocean, isotope surveys (14C‐CH4,δ13C‐CH4, δ2H‐CH4) were conducted at five locations: Skan Bay (SB), Santa Barbara Basin (SBB), Santa Monica Basin (SMB), Cariaco Basin (CB), and the Guaymas Basin (GB). Depth distributions of CH4 concentration and isotopic abundance were determined for both the sediment and water column at the SB, SBB, SMB, and CB sites; CH4 emitted from seeps on the continental shelf adjacent to the SBB as well as seeps and decomposing clathrate hydrates in the GB was also collected, purified, and analyzed. Methane isotope distributions in the sediments were consistent with known methanogenic and methanotrophic activity; seep‐ and clathrate‐hydrate‐derived CH4 was found to be depleted in radiocarbon. However, surprising results were obtained in the water column at all sites investigated. In SB the radiocarbon content of the subsurface CH4 concentration maximum was on average 41% less than its suspected sediment CH4 source, suggesting CH4 seepage in the bay. In the SBB, SMB, and CB, the 14C‐CH4 contents in the subsurface ocean were 1.2 to 3.6 times greater than modern carbon quantities suggesting a source of 14C from atmospheric nuclear weapons testing, nuclear power plant effluents, or cosmogenic isotope production.

Emission of methane from chalk streams has potential implications for agricultural practices

Summary1. The emission of biogenic gases, particularly methane, is usually associated with wetlands rather than clean streams. Here, we investigated methane production from a southern English chalk stream, where increased sedimentation, compounded by extensive macrophyte growth, may have altered ecosystem function.2. Cover of the channel by the dominant macrophyte, Ranunculus penicillatus, peaked in August, when plant beds were associated with low water velocity and the accumulation of sediment (&lt;2000 μm) dominated by the sand‐sized fraction (63–1000 μm).3. Over spring and summer there was a marked increase in the silt/clay fraction of the sediment, a concomitant drop in mean particle size and, hence, inferred permeability. At the same time there was an increase in CH4 transport through Ranunculus stems and an increase in water column CH4 concentration, while the sediment CH4 concentration increased 100‐fold between February and April. A marked seasonal enrichment in the δ15N of N2 dissolved in the pore water correlated with CH4 flux and, coupled to the shift in particle size, suggested a transient input of organic matter, possibly of terrestrial origin.4. Potential areal methane production and measured efflux were similar to that from some U.K. peatlands and represent one of the first accounts of significant methanogenesis to be measured in a stream channel. Most (&gt;90%) of the methane flux is transported to the atmosphere through the Ranunculus stems.5. Although the total flux of methane from U.K. chalk streams is probably relatively modest (estimated at 3.2 × 10−6 Tg CH4 year−1), this phenomenon changes our perception of the health of these ecosystems and indicates another deleterious side effect of agriculture.

Fossil methane source dominates Cariaco Basin water column methane geochemistry

Natural radiocarbon measurements on methane (14C‐CH4) extracted from the Cariaco Basin water column show that 98% of the methane in Cariaco Basin waters is derived from fossil (radiocarbon‐free) sources. Previous work on Cariaco Basin methane (CH4) considered only a diagenetic sediment source. Similar measurements of sediment 14C‐CH4 indicate that sediment CH4 is produced from modern particulate material; thus the sediment and water column CH4 have distinct sources. Using time‐dependent CH4 geochemical box models which include a fossil seep CH4 source term, we estimate 1) 0.024–0.028 Tg y−1 of seep CH4 are added to the Cariaco Basin water column, 2) the water column CH4 will reach a steady‐state concentration by the year 2065, and 3) the seep CH4 inputs possibly began in 1967, following the July 30 Caracas earthquake. Oxidizing this CH4 to dissolved inorganic carbon does not appear to affect Cariaco Basin 14C chronologies.

Diel Variation of Methane Fluxes in Summer in a Eutrophic Subtropical Lake in China

ABSTRACT The summer diel variation of methane (CH4) flux was investigated in a eutrophic, subtropical lake in China. The CH4 concentration was always supersaturated, and the emission rate ranged from 0.24 to 45.51mg m−2h−1. The diel variations of CH4 flux in June and August showed a single peak in early afternoon and a minimum in the morning, while the pattern varied irregularly in May. There was a moderate relationship between water and sediment temperature and CH4 emission rate in some months.

Adsorption as a methane storage process in natural lake sediment

Methane stored in a natural lake sediment was investigated. Fresh sediment (8–25 cm in sediment depth) taken from Lake Biwa, Japan, contains 2.6 μmol/g d.s. (dry sediment) of CH4 on average. The sediment retained 0.31 μmol/g d.s. of CH4 even after air drying, which is 12% of the total CH4 in the fresh sediment. The higher the temperature for drying of the sediment, the smaller the amount of CH4 retained. The result of an adsorption experiment indicated that dried and completely degassed lake sediment adsorbs CH4. Clay particles in the sediment are possibly responsible for the adsorption of CH4. Methane observed in the fresh lake sediment was probably produced by bacteria in the past, then adsorbed on the sediment particles. Adsorption is another form of storage of CH4 in lake and ocean sediments, in addition to the well known forms of dissolved, bubble, and hydrate CH4.